The U.S. Pat. No. 5,392,744 to Regueiro, entitled “Precombustion Chamber for a Double Overhead Camshaft Internal Combustion Engine” which issued Feb. 28, 1995, discloses a prechamber construction for a diesel engine in which a plurality of transfer passages are formed in the bottom of the prechamber. The passages are circumferentially spaced about the prechamber's centerline and the longitudinal axis of each passage is inclined with respect to the centerline to provide rotation of the air about the centerline. Each passage is tapered with a narrow end opening to the prechamber and a larger end opening to the engine's combustion chamber. The two ends of a passage extend circumferentially about the prechamber's centerline and situated about 90 degrees from one another. One purpose of the tapered and circumferentially aligned passages is to induce swirl first within the prechamber as air is introduced by upward movement of an associated piston in the engine's cylinder. Another purpose is to distribute and diffuse products of combustion from the prechamber into the combustion chamber and specifically direct flows toward the pockets formed in the piston for valve relief.
The combustion system described in the '744 patent was primarily useful for smaller, high-speed diesel engines as would be used on automobiles and light trucks. It was intended to facilitate combustion in the combustion chamber near the periphery of a piston. This is common in a majority of modern open-chamber type (DI or direct injection) diesels. On these relatively small engines it is difficult to locate the uppermost piston ring very close to the upper surface or crown of the piston as is desirable to minimize the annular volume formed between the piston and cylinder wall above the piston ring referred to as the headland height. Reduction of the headland height without provision for cooling the adjacent piston volume and cylinder wall tends to cause the piston ring to operate at very high temperatures. At high temperatures, the ever present soot accumulates in the ring grooves and can be “coked” thus cause the rings to stick in the grooves and to lose their sealing effectiveness which requires an engine overhaul to correct.
Piston ring overheating partially stems from the inherent design of the aforedescribed small diesel engines. They typically have cylinder liners integrally formed in the parent metal of the block and they usually have a closed-top block type design resulting in a thick top deck of the block. These factors are detrimental to maximizing cooling of the upper portion of the cylinder bore wall. Ring temperatures are further increased by the relative thickness of the upper portion of the piston's periphery necessitated to withstand the pressure-generated forces of combustion on the attendant high temperatures. The piston's thickness at its upper periphery where the rings are mounted limits the cooling effectiveness of the engine oil within the piston interior.
A solution for preventing a piston ring from attaining an undesirably high temperature is to increase the headland height which is effective to prevent coking of the piston ring in its groove. Of course, the peripheral portion of a typical aluminum piston above the uppermost ring must be formed with a sufficiently decreased diameter relative to the cylinder wall to allow for the maximum thermal expansion of the piston to prevent rubbing. Thus, increasing the headland height causes the piston/cylinder wall clearance to be increased which undesirably results in a larger headland volume of cooler trapped air. This is a particular concern when a diesel is started in a cold environment where the contraction of the aluminum piston increases the headland clearance and creates such a large volume of trapped air that the heat of compression at the injection point is so absorbed that starting is inhibited. It is believed that as much as twenty-five percent of the air can escape to the relatively cool headland clearance under cold start conditions. Contact of this large volume of air with the cool walls of the cylinder and piston increases the air's density and mass which robs the central portion of the combustion chamber of an equivalent mass of air. This air in the headland cavity or crevice is referred to as “inactive air” as opposed to the air in the remainder of the combustion chamber which is “active air”. Under cold starting conditions, the ratio of active to inactive air decreases and with it the compression pressure and temperature of the active air decreases. This factor undesirably limits the minimum starting environmental temperature for the engine.
On any engine, the surface-to-volume (S/V) ratio in a combustion chamber when at TDC during a compression stroke is an inverse function of the cylinder displacement. Small engines have high S/V ratios and larger engines have lower S/V ratios. This effect is discussed in SAE Paper 940205 authored by applicant and Salo J. Korn, and entitled “Geometric Parameters of Four Valve Cylinder Heads and Their Relationship to Combustion and Engine Full Load Performance.” This paper was presented in Detroit, Mich. at the 1994 SAE International Congress and Exposition meeting Feb. 28 to Mar. 3, 1994. The higher the S/V ratio, the higher the heat loss from the chamber to engine coolant. The cold start problem is aggravated by and linked to the higher compression ratios typically used for relatively small diesel engines. Higher compression ratios mathematically increase the S/V ratio by reducing the clearance volume relative to the remainder of the chamber. The air being compressed in a cold engine having a high S/V ratio and a high compression ratio when started in a cold environment looses a very significant quantity of its temperature. This is compounded by the effect of a large amount of inactive air. Further, prechamber type engines have additional losses incurred by the high S/V ratio of the prechamber. Starting at increased elevations or under cold ambient temperatures increase the difficulty. Thus, it is understandable why so many small diesels utilize a glow-plug starting aid. The '744 patent discloses an optional heating element to solve cold-startability problems.
The prechamber described in the '744 patent does not specify a specific thickness for the bottom wall of the prechamber tip through which the transfer passages extend. Work with this prechamber design indicates that the relatively thick bottom indicative of the views in the '744 patent are quite excessive and unnecessary. This unnecessarily led to the specification that the circumferential spacing between the smaller end and the larger end of a passage was rotated about 90 degrees. The illustrated relative thickness of the prechamber bottom or tip also implied long transfer passages requiring a smaller opening to the prechamber than otherwise would be provided. The small opening of the transfer passage does desirably produce strong swirl action but also undesirably inhibits the flow of air into the prechamber and the products of combustion out of the prechamber and into the combustion chamber. Thus, it could be judged that the prechamber as illustrated and described in the '744 patent is thermodynamically unbalanced, favoring one of its functions (swirl production) versus a second important function, i.e., effective filling of the prechamber with air and discharge of the products of combustion. Therefore, as it becomes necessary to enlarge the minimum cross-section of a transfer passage and shortening its length, the 90 degree circumferential spacing of the ends of the transfer passage of necessity change. If the 90 degree spacing is maintained, the desirable alignment of the transfer passage with a combustion pocket or valve relief in the piston would not be formed.
The '744 patent did not specify the configuration of the portion of the prechamber directly between the main interior volume and the bottom tip wall, commonly referred to as the “throat”, simply assuming it to be a straight section as is common practice. While such a throat section is necessary if a newly designed prechamber is intended to be used as a retrofit in an older engine in which the original prechamber was threadably installed in the cylinder head, this need not be the case when a newly designed prechamber is intended to be used in a newly designed cylinder head. In such a case, it is preferable to eliminate the throat portion, which is restrictive to air flow, and blend the walls of the main prechamber directly with the tip portion. This allows a conical design for the main body of the prechamber which reduces its S/V ratio and its heat losses. Also, it is simpler, lighter, and cheaper to make. As a conical body cannot be threaded, the newly designed prechamber would have to be clamped to the top of the cylinder head using conventional methods. This approach allows the lower sealing seat of the prechamber to be placed anywhere along the tapered wall which is beneficial as explained hereinafter.
The subject improved combustion chamber makes use of deep bowl pistons which in their basic form were used much earlier by the open-chamber (no prechamber) type British Gardner engines and also by the later open-chamber Murphy engines from Milwaukee. Neither of these engines included internal oil cooling for the piston rings nor location of the piston rings high on the piston.